Theft of inventory or materials from commercial establishments is an ever-increasing problem. Systems attempting to mitigate such theft have been disclosed, with one such system described in U.S. Pat. No. 3,665,449 which issued May 23, 1972, and which is incorporated herein by reference. This system is representative of those known in that a "marker" element is secured to an object which is to be detected, and electrical circuits are included to detect a signal produced by that marker. The particular marker described within the above referenced system comprises a narrow ribbon of a specialized ferromagnetic material such as permalloy, which has a very high permeability (e.g., greater than 10.sup.5 gauss/oersted), and an aggregate saturation magnetic moment of at least 0.1 electromagnetic units (pole-centimeters). A first circuit of the system described applies an alternating magnetic field within a defined interrogation zone. This applied field has a predetermined peak magnitude of less than 20 oersteds, and is periodically alternating at a frequency within the audio range. When an object bearing a marker is passed into the applied field and a vector component of that field becomes oriented with a major dimension (for example, length) of the marker, the magnetization of the marker reverses at each alternation of the applied field. Each magnetization reversal within the marker produces a characteristic pulse of external magnetic field. A second circuit of the system detects this pulse, thus verifying the presence, identity, or status of the object.
It is characteristic of the ferromagnetic material utilized within this system that even the slightest mechanical strain applied to the marker tends to cold work the material and degrade its permeability and other magnetic properties (see U.S. Pat. No. 3,665,449, Col. 6, lines 4-12). For this reason, it has been customary to install the marker within a rigid object, or to protect the ferromagnetic material within cover means which reduce the amount of bending or twisting. These cover means, typically molded plastic housings, tend to be bulky and of considerable weight as compared to the ferromagnetic material within. The undesirable weight and bulkiness of such a marker has severely restricted the use of the system with fragile and deformable items like clothing, wherein besides being aesthetically unattractive, a heavy marker hanging thereon could actually stretch the fibers of the garment and affect its saleability. Hence, a lightweight marker is desirable but can be used only if it can still limit mechanical strain from being applied to the ferromagnetic material therein.
One known technique disclosed in U.S. Pat. No. 4,342,904 (Onsager) for avoiding the degradation of the magnetic properties of such a strain-sensitive ferromagnetic marker is to encompass the ferromagnetic material between a pair of thin, lightweight sheets of a fairly rigid material, and wherein a low-friction release paper is provided between at least one of the cover sheets and one face of the magnetic strip, to afford slippage between the strip and that cover sheet, thus allowing some bending of the assembly without the transfer of mechanical strain to the ferromagnetic strip. In addition, such a marker also includes a layer of pressure-sensitive adhesive coated foam tape placed adjacent at least one face of the ferromagnetic strip, which tape serves to further cushion the ferromagnetic strip from mechanical strain and stresses.
It is also known to provide similar markers wherein the ferromagnetic material is substantially immune to work hardening. As disclosed in U.S. Pat. No. 4,298,862 (Gregor et al.), such a material may be a ductile strip of amorphous metal such as that manufactured by Allied Corporation under the trademark "Metglas". While such a material is nominally ductile as defined in that patent, the material is nonetheless relatively springy, and in thin sheet form has sharp edges which restrict the use thereof directly on or in cloth or fabric articles.